1. Field of the Invention
The present invention relates to medical devices dealing with cardiovascular ailments. More particularly, the field of the invention is that of fibrillation detectors and defibrillators.
2. Description of the Related Art
Implantable cardioverters, or defibrillators, are known which monitor electrical activity to determine the presence of tachycardia or fibrillation in the ventricles of the heart and to deliver an electrical therapy to terminate the tachycardia. These prior art devices monitor electrical activity in the heart from a single pair of electrodes. For example, U.S. Pat. No. 4,384,585 to Zipes, entitled SYNCHRONOUS INTRACARDIAC CARDIOVERTER, the disclosure of which is explicitly incorporated by referenced herein, discloses an implantable medical device which senses tachyarrhythmias and delivers cardioverting pulses. Ventricular fibrillation is conventionally determined by observing the time interval in which the electrocardiogram (ECG) signal is at a baseline the determination of which is based on a probability density function. Another conventional method involves observing a shortened cycle length that is less than a cut-off value. The probability density function is often insufficiently sensitive to detect ventricular arrhythmia, while the shortened cycle length often results in overlapping with other arrhythmias such as sinus tachycardia or atrial fibrillation with a rapid ventricular response.
A difficulty in detecting fibrillation based on cycle length is that the observed characteristic does not uniquely identify the underlying arrhythmia. One problem with fibrillation detectors is that incorrect therapy may be delivered to patients with ECG readings that mimic ventricular tachycardia or fibrillation on the basis of cycle lengths. Although the determination of the exact type of arrhythmia may be improved by improving the probability density function and/or the rate cut-off detection approaches, greater specificity is desired to minimize the chance of making a false determination.